Re: Hash table performance
On Sun, 22 Nov 2009, markspace wrote:
Kevin McMurtrie wrote:
Yes. Creating custom hashing classes for primitives pays off if
performance needs to be very high.
I've actually been playing around with "alternate ways of handling
collisions" and I agree that hashing a double is hard. The default
algorithm both Tom and I used:
long bits = Double.doubleToLongBits( key );
int hash = (int)(bits ^ (bits >>> 32));
provides terrible performance.
Interesting. I chose that function because it's what java.lang.Double
does, rather than because i thought it'd be good, but i am surprised to
hear it's terrible - doubles are quite complicated internally, so would
have thought that a parade of natural numbers would give reasonably
well-distributed hashes this way (whereas longs wouldn't, of course). How
did you conclude it's terrible? Can you reccommend a better hash function
for doubles?
tom
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